View Full Version : Pressurization Question
20th Jul 2002, 21:27
How does the pressurization work on commercial aircrafts. Is it automatic once the doors are closed or does it start once you are above a certain altitude?
Also what does the command - "all doors to automatic" and "all doors to manual" signify.
It starts a slight pressure once the doors are closed keeping in mind the need for 'evacuation'.....The Airbus 320 cabin starts to climb above 5040'agl,allowes the a/c return to field..
The call's doors to manual apply after landing to avoid the Auto system from opening the doors under press,and inflating the chutes...'to Auto' apply once the acraft taxiis-in case of the evacuation.....
21st Jul 2002, 05:10
"Doors to Automatic" is a command for the cabin to "arm" the doors for automatic slide-deployment, in the event the doors were opened in an emergency. This is normally issued just before the aircraft leaves the parking gates, after all the doors are closed.
"Doors to Manual" is the command to "dis-arm" the door slides, usually after the flight arrives at the destination airport, and just before the aircraft gets to the parking gate
21st Jul 2002, 19:31
... or, given the usual UK policy of having crew remaining seated until the aircraft has stopped (like the pax), when the seat belt sign is switched off.
Aircraft pressurisation is an incredibly complicated subject, and varies with different aircraft type, and on each type there are probably two or three systems anyway. In broad terms, however, in most cases you set the cruise altitude and landing altitude (either overhead or via the FMC/FMGC), and the aircraft works out a pressurisation schedule for itself.
23rd Jul 2002, 02:19
"How does the pressurization work on commercial aircrafts."
The airconditioning units ("packs") provide pressure by pumping air into the cabin. A valve (or several valves) controls the outflow of air from the cabin. The valve is usually found towards the rear of the aircraft, on the underside of the fuselage. To increase pressure, the valve moves towards close. To decrease pressure, the valve moves towards open.
Doors "AUTOMATIC" and "MANUAL" is a Boeing term
for arming or disarming the door slide system, in my opinion a confusing set of terms used. There is more than one flight attendant out there that got those two terms mixed up and dumped a slide pack.
On the Bus its "ARMED" and "DISARMED". A little more straight forward.
To feed Quavian's( excellent explanation) Packs-bleed air is taken from the engine;)
26th Jul 2002, 20:11
From a flight deck visit (pre 9/11) I was told the Mr Boeing's NG machines will respond to the instructions mentioned by Young Paul, during the take off roll.
When it sees the speed build, it presumes that a departure is in the offing (smart things these computers ;) ) and starts to put us all under pressure, as described by QAVION.
26th Jul 2002, 21:15
The Airbuses take the application of FLX or TOGA as signal for departure, and start pressurizing there, usually to be noticed around 70 or so kts, because then the valve mentioned above has reached a position where pressure can build up.
28th Jul 2002, 19:45
Fortunately, and I say that in all seriousness, the pressurization system in the B747 Classic is controlled manually, by the Flight Engineer.
He sets the intended initial cruise altitude prior to engine start, but the system doesn't start anything at all until after the main wheels lift off at take-off. The two main cabin outflow valves, located at the rear of the fuselage, underside, then start to close automatically, to initiate the pre selected rate of cabin altitude climb, usually about 300 feet/min, but can be adjusted as req, by the FE.
The FE selects a cruise altitude of 1000 feet above that of the cruising aircraft, for passenger comfort, so that any small fluctuations of aircraft altitude, which occur all the time on all aircraft, don't cause any small fluctuations in cabin altitude, which can be quite annoying on the eardrums of all aboard.
The FE will select a slower rate of cabin climb when the aircraft is very heavy and climbing more slowly, again, for pax cpmfort.
On descent, the FE selects the runway altitude, and again controls the rate of decent, to both arrive at that altitude together. The system will not allow you to reach the far end of the landing runway with the cabin still pressurized, for safety.
This entire sequence of events can be controlled fully manually, should that be required, then you will see an expert at work!!
There are all sorts of alternate procedures the FE has in his head to regain control of the cabin pressure, should a malfunction occur, which is very rare.
A very good system, under the trained eye of the Flight Engineer, with an excellent track record.
Beats a computer controlled, automated system anytime!!!
29th Jul 2002, 03:12
... and, FD, don't forget that you can also have the APU running to operate the #2 pack for take-off on very hot days (With the isolation valves closed) , then swap it all over to engine bleed only after take-off. ;)
Ah...the Flight Engineer. That fellow who can save your bacon a time or two (and do the walk-around in the rain)...the younger guys will NEVER know the benefits:D :D :D
29th Jul 2002, 08:08
The hardest working bloke in the plane.